The use of highly active antiretroviral therapy (HAART) can reduce HIV viremia to nearly undetectable levels in infected individuals through suppression of the viral lifecycle, however there are increasing problems associated with long-term toxicity, therapeutic compliance, high cost and the emergence of resistant strains.1,2 In addition, suppressive treatment strategies lead to the formation of latent reservoirs of low-level HIV-1 replication. Upon treatment cessation, these persistent sources lead to rapid HIV-1 rebound;3-5 thus, strict adherence to rigorous life-long treatment is required. Given these major barriers, new therapeutic strategies that are capable of eliminating these persistent reservoirs are critical to eradication of HIV infection.
Currently, one of the most investigated strategies aimed to cure HIV infection is the development of an HIV vaccine, however, as discussed in Chapter 2, major barriers have thwarted these efforts in the past quarter century. These barriers include HIV's enormous genetic diversity and propensity for genetic recombination, its detrimental toll on the immune system through the destruction of T cells, HIV's highly-evolved immune evasion strategies, and lastly, the establishment of latent reservoirs in which HIV is immunologically silent.6 The major feature of HAART is its ability to block HIV replication and prevent new infection, however it fails at killing cells that are already infected. Thus, strategies that complement HAART-induced suppression by directly killing infected cells (including elimination of latent reservoirs) represent enormous potential towards efforts to cure HIV.7 
Targeting HIV-Infected Cells with Cytotoxic Conjugates
Because Env resides on the surfaces of free virions and infected cells, and also mediates virus entry into host cells, numerous strategies for cytotoxic targeting of Env-expressing cellular reservoirs have been investigated. As discussed throughout the entirety of this thesis, ARM-Hs have the promise of being such a strategy, as we have demonstrated their success at not only inhibiting viral fusion, and therefore suppressing replication, but also at targeting HIV-1 gp120 expressing cells for immune-mediated toxicity. In addition, pioneering work focused on developing “immunotoxins,” Env-targeting protein constructs conjugated to potent cellular toxins. Berger and colleagues published the first example of this class with their gp120-targeting CD4-Pseudomonas exotoxin A (PE) chimera, CD4-PE40.8 Significant gp120-specific cellular toxicity (of both Env-transfected cells and constitutively HIV-infected cell lines) and inhibition of spreading infection by CD4-PE40 was demonstrated in vitro.9-12, 96 This early success made it the only immunotoxin to enter Phase I clinical trials, where it failed due to a complete lack of antiviral activity at the maximum dosage which was limited by severe hepatocellular injury. 13-15 More recently, Pastan and co-workers conjugated PE to the single-chain Fv fragment of the broadly neutralizing 3B3 anti-gp120 antibody, yielding an antibody-toxin chimera, PE38, which demonstrated potent specific toxicity to Env-transfected cells as well as a chronically HIV-infected lymphocytic cell line.11, 16 Importantly, PE38 demonstrated 20-30-fold more effective killing of HIV-infected cells than CD4-PE40 and is hypothesized to possess significantly less hepatotoxcity.11, 17 Most recently, it has been demonstrated that these chimeras dramatically augment the antiviral activity of HAART in thy/liv-SCID-Hu mice,17 providing promising evidence that such synergistic clinical strategies may indeed lead to an eradication strategy for HIV infection.7 Other notable examples include work by Root et al. who recently developed a 5-Helix protein-PE chimera that binds to HIV-1 gp41, demonstrating potent cytotoxicity towards HIV-1 infected cells. In work by Johansson et. al., the potent DNA-intercalating small molecule drug doxorubicin was conjugated to an anti-gp120 mAb and then administered to mice possessing HIV-1/MuLV-infected splenocytes, resulting in the elimination of the infection.18 
Collectively, this work demonstrates the overwhelming promise of strategies to direct targeted cytotoxicity against HIV-infected cells. However, all previously reported strategies utilize protein constructs, which have shown acute toxicity and immunogenicity in cellular systems7, 19 and are inherently limited by the characteristics of all protein therapeutics.20 These limitations include the potential for life-threatening allergic reactions, poor tissue penetration, immunogenicity (even in the case of “humanized” proteins),21 lack of oral bioavailability, requirement for low-temperature storage, and high cost.22 Given the promise of such approaches, we sought to overcome their limitations by utilizing our gp120-targeting small molecule scaffold to deliver cytotoxic compounds to HIV-infected cells. In addition, such agents might prove particularly useful if antibody-mediated killing (via ARM-Hs) proves ineffective in vivo, or in patients with highly compromised immune systems.